A pulse oximeter sensor measures the percentage oxygen saturation in arterial hemoglobin, which indicates percentage of hemoglobin molecules in the arteries that contain an oxygen molecule. This measurement also is commonly referred to as saturation of peripheral oxygen, or as SpO2. FIG. 1 is an illustrative drawing representing blood oxygenation levels in an artery. In this illustrative example, seventy-five per cent of the blood hemoglobin molecules are oxygenated (HbO2) and twenty-five percent of the blood hemoglobin molecules are deoxygenated (Hb).
Oxygenated and deoxygenated blood have different absorption levels for red and infrared (IR) light. FIG. 2 is an illustrative drawing representing the difference in absorbance levels of two different light wavelengths in the red and IR bands, by oxygenated hemoglobin and deoxygenated hemoglobin. The ratio of the absorbance of oxygenated hemoglobin to deoxygenated hemoglobin is lower in the red portion of the spectrum than it is in the IR portion of the spectrum. In other words, Hb absorbs red light more readily than HbO2, and HbO2 absorbs IR light more readily than Hb.
A typical pulse oxirneter operates by directing red light emitted by a red LED and IR light emitted by an IR LED onto a patient's body tissue, and measuring the intensity of red and IR light that passes through the tissue medium and is detected by a photodector. The relative intensities of red and IR light detected by the photodetector, and how those intensities vary in response to the heartbeat pulse, provide a measure of blood oxygenation level.
Arterial volume changes in a periodic pattern in response to blood pressure variations during a heartbeat. FIG. 3 is an illustrative waveform showing a typical periodic pattern of the detected light which correlate with the changes in arterial volume during a sequence of blood pressure pulses within an artery and also showing corresponding representations of maximum and minimum arterial volumes that occur during each pressure pulse. During a systole phase of a heart beat activity, when arterial pressure is at a maximum, arterial volume and arterial diameter are at a maximum so there is more arterial blood flow and thus more light is absorbed and then, less light arrives to the photodetector. During a diastole phase of a heart beat activity, when arterial pressure is at a minimum, arterial volume and arterial diameter are at a minimum. Thus, the red and IR wavelengths absorbed by the artery has an AC pulsatile characteristic that can be isolated from the DC absorption characteristic of other components, such as tissue, non-pulsatile arterial blood, venous blood, skin reflection or even stray light, for example, whose volume does not vary in periodic pattern. A pulse oximeter sensor typically utilizes the periodic time varying nature of arterial volume to distinguish red and IR light absorption by oxygenated and deoxygenated hemoglobin within the blood from red and IR light absorption by other components and tissues surrounding or adjacent to the artery such as muscle, nerve, fat, or connective tissue, for example.
In the past, to measure both red light absorption and IR light absorption during each heartbeat, a red LED and an IR LED have been alternately turned on and off to produce alternating pulses of red and IR light. A photodiode detects alternating pulses of red and IR. light that have passed through the arteries, arterioles, and capillaries that transport arterial blood. Very often the light penetrates in the tissue but does not really get to the artery. Rather, it diffuses though the capillaries closer to the skin which have a similar (but softer) behaviors to what has been described for arteries. SpO2 level is determined based upon both the average (constant) and pulsatile (that which varies in response to the heartbeat) relative-intensities of red and IR radiation detected. FIGS. 4A-4B are illustrative waveforms 402, 404 representing typical evolution of arterial volume during a sequence of blood pressure pulses within an artery (FIG. 4A) and an alternating sequence of current flow pulses in a red LED and an IR LED turn-on signals (FIG. 4B) to alternately turn-on the red LED and the IR LED. A sequence of first current pulses 408 having a first current value are provided during a sequence of first time intervals to turn-on the red LED during each of the first time intervals. A sequence of second current pulses 410 having a second current value are provided during a sequence of second time intervals to turn-on the IR LED during each of the second time intervals. The first time intervals and the second time intervals are interleaved so that the red LED and the IR LED take turns, or alternate, turning on to produce an alternating sequence of red and IR light pulses. Photodetector measurements of the diffused light produced by the red and IR LEDs are processed to determine red and IR light absorption levels.
FIG. 5A is an illustrative drawing showing a first pulse oximetry system 500 that is powered from an external power source. The first system 500 includes a housing 502 that encloses a pulse oximetry sensor (not shown). In operation, the housing is mounted on a patient's finger to position the sensor to take pulse oximetry measurements. A wire 504 couples the sensor with signal processing circuity 506 that includes a user interface screen 507 to display pulse oximetry measurements. The sensor sends signals over the wire 504 to the processing circuitry 506 that are indicative of red light intensity and IR light intensity incident upon a photodiode (not shown). The wire also provides power to the sensor from the external power source. The sensor also receives LED control signals over the wire 504 from the processing circuitry 506 to control the intensity of light emitted by the red and IR LEDs, The processing circuity 506 is mounted on a portable work station 508.
FIG. 5B is an illustrative drawing showing a second pulse oximetry system 520 that is battery powered. The second system 520 includes a housing 522 that encloses a pulse oximetry sensor (not shown). In operation, the housing 522 is mounted on a patient's finger so as to position the sensor to take pulse oximetry measurements. A wire 524 couples the sensor with a battery powered smart watch 526 that includes a user interface screen 527 configured to display pulse oximetry measurements. The wire also provides power to the sensor from the watch battery (not shown). The smart watch 526 receives signals indicative of light intensity, performs signal processing, displays measurement results, sends control signals and is mounted on the patient's wrist.
FIG. 5C is an illustrative drawing showing a third pulse oximetry system 540 that is battery powered. The third system 540 includes a housing 542 that encloses a pulse oximetry sensor (not shown), built-in processing circuitry (not shown), a built-in display screen configured to display pulse oximetry measurements, and a battery (not shown). In operation, the housing 542 with its associated sensor, processing circuity, and display screen 547, are mounted on a patient's finger to position the sensor to take pulse oximetry measurements.
FIG. 5D is an illustrative perspective view of a fourth blood oximeter sensor system 560 that is battery powered. The fourth system 560 includes a housing 562 that encloses a pulse oximetry sensor (not shown). The sensor system 560 communicates wirelessly via RF transmissions 570 with an external processing system 566 that includes a display screen 567 to display blood oximetry measurements.